Tissue paper is well known in the art. Tissue paper is used as facial tissue, bath tissue, paper toweling, napkins, place mats, etc. The tissue paper may be provided with indicia for either aesthetic or functional purposes. Indicia may also be applied to paper plates. Typically the indicia are printed.
Consumers react positively to aesthetically pleasing indicia. Printed indicia have evolved from line printing to process printing, evoking positive consumer response. However, current process printing, without more, is not, sufficient to render high fidelity indicia on tissue substrates. High fidelity printing is necessary to reproduce, with fidelity, indicia corresponding to the source of the indicia. In the prior art, the source of the indicia has typically been artwork. Artwork is authored by an artist, may optionally correspond to a theme which is desirable to the consumer, and is commercially available from several vendors.
More recently photographs have been used as the source of the indicia. Photographs may be taken of landscapes, scenery, or even of artwork. Photographic sources of indicia comprising individual elements such as animals, flowers, etc. may be particularly desirable to consumers. Photographs comprise images which are reproduced digitally, including scanning, otherwise electronically, or chemically onto a film. It is not critical how the photograph is reproduced, only that it be reproducible to a tangible medium of expression.
However, high fidelity printing of indicia taken from a photographic source presents special challenges. If several parameters are not properly adjusted, the indicia will not have enough fidelity to appear realistic.
Recently, one vendor of commercial printing equipment executed quality printing on a tissue substrate. However, the substrate was apparently conventionally dried on a press felt. Printing high fidelity indicia on a conventionally dried substrate is a relatively easy task because such a substrate is smooth. But consumers often desire through air dried substrates, which are textured. However, printing high fidelity indicia on a textured substrate is more difficult due to the rugosities and asperities inherent in the substrate.
Successful printing of high fidelity indicia on a textured substrate does not directly translate from experiences printing such indicia on a smooth substrate. Several complications occur. For example, textured through air dried substrates are typically more absorbent. This increased absorbency results in dot gain, causing dots of ink to run together, blurring the image represented by the indicia. The dot gain may be accommodated by decreasing the amount of ink used to print the indicia. However, a uniform downward adjustment of all colors will result in indicia having a muddy appearance or even a faded and washed out appearance, if enough color is removed. Such adjustments can either be done electronically with software or by adding less ink to the printing roll.
Compensating for dot gain on a tissue paper substrate is made particularly difficult by the nonlinear response of the dot gain as a function of the amount of area collectively covered by the dots. Referring to FIG. 1, it is seen that the dot gain increases as less area of the tissue paper is desired to be covered. However, the nonlinear response does not permit easy compensation for the dot gain by decreasing the amount of ink alone.
Another approach to compensating for the increased absorbency of a textured substrate is to try color purification. Color purification results from using fewer primary colors to achieve the desired shade. For example, four color printing may use magenta, yellow, cyan, and black to produce a spectrum of colors. Alternatively, four color printing may utilize brown, green, blue and black, provided red and orange shades are not desired. If one used an evenly reduced mix of the four process colors to achieve the same shades as was produced using more colors, less total ink could be applied to the textured substrate. However, color purification, without more, results in indicia having a faded and washed out appearance.
In yet another approach, one could increase the resolution and decrease the volume of the anilox roll used to print the indicia if flexographic printing is used. However, this approach has been shown to result in less than full coverage of ink occurring where the indicia is desired, and requires higher pigment concentrations in the ink.
It is clear that there is a need in the art for an improved printing process, particularly one which yields high fidelity printed indicia on a textured substrate. There is further a need for such a process which can be used with photographically sourced indicia. Such a process, and the printed tissue paper produced thereby are described and claimed below.
The present invention is also applicable to printing on other textured substrates. For example, textured substrates may be used in packaging to provide surfaces which are easily gripped by the user. Further, the substrate need not be cellulosic, if other material properties are desired.